Color distribution in exit pupil expanders

ABSTRACT

The specification and drawings present a new apparatus and method for providing color separation in an exit pupil expander system that uses a plurality of diffractive elements for expanding the exit pupil of a display in an electronic device for viewing by introducing a selectively absorbing area or areas in the exit pupil expander.

PRIORITY AND CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/227,738 filed Oct. 19, 2009, which was a national phase entry ofInternational Application No. PCT/IB2006/001450 filed Jun. 2, 2006.

TECHNICAL FIELD

The present invention relates generally to a display device and, morespecifically, to providing color separation in exit pupil expanders thatuse a plurality of diffractive elements for expanding the exit pupil ofa display for viewing.

BACKGROUND ART

While it is a common practice to use a low-resolution liquid-crystaldisplay (LCD) panel to display network information and text messages ina mobile device, it is preferred to use a high-resolution display tobrowse rich information content of text and images. A microdisplay-basedsystem can provide full color pixels at 50-100 lines per mm. Suchhigh-resolution is generally suitable for a virtual display. A virtualdisplay typically consists of a microdisplay to provide an image and anoptical arrangement for manipulating light emerging from the image insuch a way that it is perceived as large as a direct view display panel.A virtual display can be monocular or binocular.

The size of the beam of light emerging from imaging optics toward theeye is called exit pupil. In a Near-Eye Display (NED), the exit pupil istypically of the order of 10 mm in diameter. Further enlarging the exitpupil makes using the virtual display significantly easier, because thedevice can be put at a distance from the eye. Thus, such a display nolonger qualifies as a NED, for obvious reasons. Head-Up Displays areexamples of the virtual display with a sufficiently large exit pupil.

DISCLOSURE OF THE INVENTION

According to a first aspect of the invention, an apparatus, comprises:at least one substrate of optical material having a first surface and asecond surface; at least one diffractive element disposed on the firstor the second surface of the at least one substrate and configured toreceive an input optical beam; at least one further diffractive elementdisposed on the first or the second surface; and at least one opticallyabsorbing area within or disposed on the at least one substrate, whereinat least part of the input optical beam is diffracted in the diffractiveelement to provide at least one diffracted optical beam substantiallywithin the first and the second surfaces, and at least part of the atleast one diffracted optical beam is coupled out of the at least onesubstrate by diffraction in the at least one further diffractiveelement, thus providing an output optical beam out of the at least onesubstrate with an expanded exit pupil in one or two dimensions, whereinthe input optical beam comprises K wavelength components and the atleast one optically absorbing area is configured to absorb Mpre-selected components out of the K wavelength components of the atleast one diffracted optical beam and to propagate a selected componentout of the K wavelength components of the at least one diffractedoptical beam, such that the output optical beam comprises the selectedcomponent out of the K wavelength components, wherein K is an integer ofat least a value of two and M is an integer between 1 and K−1.

According further to the first aspect of the invention, the opticaldevice may comprise N stacked substrates of optical material separatedby a gap, N being an integer of at least a value of one, and the atleast one substrate being one of the N substrates, wherein each of the Nstacked substrates is configured to expand an exit pupil substantiallyof only one component out of the K wavelength components of the inputoptical beam such that all the K wavelength components merge togetherhaving substantially the same direction and location in an output of theoptical device. Further wherein N=K.

According further to the first aspect of the invention, wherein M may beequal to K−1.

Still further according to the first aspect of the invention, the outputoptical beam provided by the at least one substrate substantially maycomprise only the selected component out of the K wavelength components.

According further to the first aspect of the invention, the at least oneoptically absorbing area may be configured using at least one of: a) anabsorbing tint spread throughout a volume of the at least one substrate,b) an absorbing tint spread throughout a thickness of the at least onesubstrate only in areas of the at least one substrate between the atleast one diffractive element and at least one further diffractiveelement, c) an absorbing tint spread throughout a resin used to make theat least one diffractive element, d) an absorbing tint spread throughoutthe resin used to make the at least one further diffractive element, ande) an absorbing coating, having substantially the same index ofrefraction as the at least one substrate, disposed on the at least onesubstrate in an area substantially between the at least one diffractiveelement and the at least one further diffractive element.

According still further to the first aspect of the invention, theapparatus may further comprise: a further substrate of optical materialhaving a further first surface and a further second surface, the furthersubstrate being stacked substantially in parallel with the at least onesubstrate but maintaining a gap with the at least one substrate; atleast one additional diffractive element disposed on the further firstor the further second surface of the further substrate and configured toreceive a portion of the input optical beam which propagates through theat least one substrate into the further substrate; at least one stillfurther diffractive element disposed on the further first or the furthersecond surface; and at least one further optically absorbing area withinor disposed on the further substrate, wherein at least further part ofthe portion of the input optical beam is diffracted in the additionaldiffractive element to provide at least one further diffracted opticalbeam substantially within the further first and the further secondsurfaces, and at least further part of the at least one furtherdiffracted optical beam is further coupled out of the further substrateby diffraction in the at least one still further diffractive element,thus providing a further output optical beam out of the furthersubstrate with an expanded exit pupil in one or two dimensions, thefurther output optical beam having substantially the same direction andlocation as the output optical beam, wherein the at least one furtheroptically absorbing area is configured to absorb P pre-selectedcomponents of the K wavelength components and to propagate a furtherselected component out of the K wavelength components, such that thefurther output optical beam comprises the further selected component outof the K wavelength components, wherein P is an integer between 1 andK−1. Further, wherein P is equal to K−1. Still further, the at least onefurther optically absorbing area may be configured using at least oneof: a) an absorbing tint spread throughout a volume of the furthersubstrate, b) an absorbing tint spread throughout a thickness of thefurther substrate only in areas of the further substrate between the atleast one additional diffractive element and at least one still furtherdiffractive element, c) an absorbing tint spread throughout a resin usedto make the at least one additional diffractive element, d) an absorbingtint spread throughout the resin used to make the at least one stillfurther diffractive element, and e) an absorbing coating, havingsubstantially the same index of refraction as the further substrate,disposed on the further substrate between the at least one additionaldiffractive element and the at least one still further diffractiveelement. Yet still further, the gap may be an air gap. Still yetfurther, the output optical beam provided by the at least one substratesubstantially may comprise only the selected component out of the Kwavelength components and the further output optical beam provided bythe further substrate substantially may comprise only the selectedfurther component out of the K wavelength components and the outputoptical beam and the further output optical beam merge together havingsubstantially the same direction and location.

Further still, the at least one substrate may be configured to have anadditional optically absorbing layer disposed on or adjacent to asurface of the at least one substrate, the surface being a secondsurface through which the input optical beam propagates, such that theselected component out of the K wavelength components is substantiallyabsorbed in the additional optically absorbing layer.

According further still to the first aspect of the invention, theapparatus may further comprise at least one intermediate diffractiveelement such that the at least part of the input optical beam diffractedin the at least one diffractive element is first coupled to the at leastone intermediate diffractive element, which then couples, using afurther diffraction in the at least one intermediate diffractiveelement, the at least part of the diffracted optical beam to the atleast one further diffractive element, which then provides atwo-dimensional exit pupil expansion of the input optical beam.

According to a second aspect of the invention, a method, comprises:

receiving an input optical beam by at least one diffractive elementdisposed on a first or a second surface of at least one substrate, theinput optical beam comprises K wavelength components, wherein K is aninteger of at least a value of two; diffracting at least part of theinput optical beam in the at least one diffractive element to provide atleast one diffracted optical beam substantially within the first andsecond surfaces; absorbing M pre-selected components out of the Kwavelength components of the at least one diffracted optical beam in atleast one optically absorbing area within or disposed on the at leastone substrate and propagating a selected component out of the Kwavelength components of the at least one diffracted optical beamsubstantially without an optical intensity attenuation in the at leastone optically absorbing area, wherein M is an integer between 1 and K−1;and coupling at least part of the at least one diffracted optical beamout of the at least one substrate by diffraction in the at least onefurther diffractive element, thus providing an output optical beam outof the at least one substrate with an expanded exit pupil in one or twodimensions, wherein the output optical beam comprises the selectedcomponent out of the K wavelength components.

According further to the second aspect of the invention, the outputoptical beam provided by the at least one substrate substantially maycomprise only the selected component out of the K wavelength components.

Further according to the second aspect of the invention, the at leastone optically absorbing area may be configured using at least one of: a)an absorbing tint spread throughout a volume of the at least onesubstrate, b) an absorbing tint spread throughout a thickness of the atleast one substrate only in areas of the at least one substrate betweenthe at least one diffractive element and at least one furtherdiffractive element, c) an absorbing tint spread throughout a resin usedto make the at least one diffractive element, d) an absorbing tintspread throughout the resin used to make the at least one furtherdiffractive element, and e) an absorbing coating, having substantiallythe same index of refraction as the at least one substrate, disposed onthe at least one substrate in an area substantially between the at leastone diffractive element and the at least one further diffractiveelement.

According to a third aspect of the invention, an electronic device,comprises:

-   -   a data processing unit;    -   an optical engine operatively connected to the data processing        unit for receiving image data from the data processing unit;    -   a display device operatively connected to the optical engine for        forming an image based on the image data; and    -   an exit pupil expander comprising:

at least one substrate of optical material having a first surface and asecond surface;

at least one diffractive element disposed on the first or the secondsurface of the at least one substrate and configured to receive an inputoptical beam; at least one further diffractive element disposed on thefirst or the second surface; and

at least one optically absorbing area within or disposed on the at leastone substrate, wherein

at least part of the input optical beam is diffracted in the diffractiveelement to provide at least one diffracted optical beam substantiallywithin the first and the second surfaces, and

at least part of the at least one diffracted optical beam is coupled outof the at least one substrate by diffraction in the at least one furtherdiffractive element, thus providing an output optical beam out of the atleast one substrate with an expanded exit pupil in one or twodimensions, wherein

the input optical beam comprises K wavelength components and the atleast one optically absorbing area is configured to absorb Mpre-selected components out of the K wavelength components and topropagate a selected component out of the K wavelength componentssubstantially without an optical intensity attenuation, such that theoutput optical beam comprises the selected component out of the Kwavelength components, wherein K is an integer of at least a value oftwo and M is an integer between 1 and K−1.

Further according to the third aspect of the invention, the exit pupilexpander may comprise N stacked substrates of optical material separatedby a gap, N being an integer of at least a value of one, and the atleast one substrate being one of the N substrates, wherein each of the Nstacked substrates is configured to expand an exit pupil substantiallyof only one component out of the K wavelength components of the inputoptical beam such that all the K wavelength components merge togetherhaving substantially the same direction and location in an output of theoptical device. Further, wherein N=K.

Still further according to the third aspect of the invention, M may beequal to K−1.

According further to the third aspect of the invention, the outputoptical beam provided by the at least one substrate substantially maycomprise only the selected component out of the K wavelength components.

According still further to the third aspect of the invention, the atleast one optically absorbing area may be configured using at least oneof: a) an absorbing tint spread throughout a volume of the at least onesubstrate, b) an absorbing tint spread throughout a thickness of the atleast one substrate only in areas of the at least one substrate betweenthe at least one diffractive element and at least one furtherdiffractive element, c) an absorbing tint spread throughout a resin usedto make the at least one diffractive element, d) an absorbing tintspread throughout the resin used to make the at least one furtherdiffractive element, and e) an absorbing coating, having substantiallythe same index of refraction as the at least one substrate, disposed onthe at least one substrate in an area substantially between the at leastone diffractive element and the at least one further diffractiveelement.

According to a fourth aspect of the invention, an apparatus, comprises:at least one means for diffraction,

-   -   for receiving an input optical beam by at least one means for        diffraction disposed on a first or a second surface of at least        one substrate, the input optical beam comprises K wavelength        components, wherein K is an integer of at least a value of two,        and    -   for diffracting at least part of the input optical beam in the        at least one means for diffraction to provide at least one        diffracted optical beam substantially within the first and        second surfaces;

at least one absorbing means within or disposed on the at least onesubstrate, for absorbing M pre-selected components out of the Kwavelength components of the at least one diffracted optical beam by theat least one absorbing means and propagating a selected component out ofthe K wavelength components of the at least one diffracted optical beamsubstantially without an optical intensity attenuation in the at leastone optically absorbing area, wherein M is an integer between 1 and K−1;and at least one further means for diffraction, for coupling at leastpart of the at least one diffracted optical beam out of the at least onesubstrate by diffraction in the at least one further means fordiffraction, thus providing an output optical beam out of the at leastone substrate with an expanded exit pupil in one or two dimensions,wherein the output optical beam comprises the selected component out ofthe K wavelength components.

According further to the fourth aspect of the invention, the apparatusmay be an exit pupil expander.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the nature and objects of the presentinvention, reference is made to the following detailed description takenin conjunction with the following drawings, in which:

FIGS. 1 a and 1 b are schematic representations of a virtual realitydisplay (a cross sectional view shown in FIG. 1 a) having an exit pupilexpander system with stacked diffractive exit pupil expanders (a crosssectional view is shown in FIG. 1 b);

FIGS. 2 a and 2 b are schematic representations (cross-sectional views)demonstrating a color separation in an exit pupil expander system usingabsorbing tint through a volume of the substrates (FIG. 1 a) and througha thickness of selected areas of the substrates between in-coupling andout-coupling gratings, according to embodiments of the presentinvention;

FIGS. 3 a and 3 b are schematic representations (cross-sectional views)demonstrating a color separation in an exit pupil expander system usingabsorbing tint spread throughout a resin in in-coupling and out-couplinggratings, according to embodiments of the present invention;

FIGS. 4 a and 4 b are schematic representations (cross-sectional views)demonstrating a color separation in an exit pupil expander system usingabsorbing tint spread throughout a resin in out-coupling gratings andabsorbing coating across the in-coupling grating, according toembodiments of the present invention;

FIGS. 5 a and 5 b are schematic representations (cross-sectional views)demonstrating a color separation in an exit pupil expander system usingabsorbing coatings having substantially the same index of refraction asa substrate and disposed on the substrate in areas between in-couplingand out-coupling gratings, according to embodiments of the presentinvention; and

FIG. 6 is a schematic representation of an electronic device, having anexit pupil expander system, according to an embodiment of the presentinvention.

MODES FOR CARRYING OUT THE INVENTION

A new method and apparatus are presented for providing color separationin an exit pupil expander system that uses a plurality of diffractiveelements for expanding the exit pupil of a display in an electronicdevice for viewing by introducing a selectively absorbing area or areasin the exit pupil expanders. The embodiments of the present inventioncan be applied to a broad optical spectral range of optical beams butmost importantly to a visible part of the optical spectrum where theoptical beams are called light beams.

According to an embodiment of the present invention, an optical device(e.g., the optical device can be a part of a virtual reality display)can comprise N (N being an integer of at least a value of one) stackedsubstantially parallel substrates of optical material separated by a gap(the gap can be a material with a substantially smaller index ofrefraction than in the substrates, e.g., an air gap), each suchsubstrate having a first surface and a second surface and acting as anexit pupil expander. Each substrate can comprise at least onediffractive element (e.g., a diffractive grating) disposed on the firstor the second surface of said at least one substrate and which isconfigured to receive an input optical beam (i.e., at least onediffractive element acts as an in-coupling grating). It is noted thatafter propagating through the in-coupling grating of the firstsubstrate, some “unselected” wavelength components of the input opticalbeam (i.e., a portion of the input optical beam) are received by thein-coupling grating of a second substrate in a stack of the N substratesand so on. It is also noted that in a biocular system it can be one ortwo in-coupling gratings adjacent to each other for the left and for theright eye, which are configured to split the input optical beam into twosubstantially equal components in two substantially opposite directions.Each substrate can further comprise at least one further diffractiveelement (e.g., a diffractive grating) disposed on the first or thesecond surface and acting as an out-coupling grating. Again, in abiocular system it can be two out-coupling gratings symmetricallylocated in the substrate to provide out-coupling for the left and forthe right eye.

Moreover, according to an embodiment of the present invention, eachsubstrate may comprise at least one optically absorbing area or areaswithin or disposed on the substrate. Thus, at least part of the inputoptical beam can be diffracted in the diffractive element (thein-coupling grating) to provide at least one (two in the biocularsystems) diffracted optical beam substantially within the first and thesecond surfaces due to a total internal reflection, and then at leastpart of the at least one diffracted optical beam is further coupled outof each substrate by diffraction in the at least one further diffractiveelement (the out-coupling grating), thus providing an output opticalbeam (or two beams for the biocular systems) out of each substrate withan expanded exit pupil in one or two dimensions using wavelengthselectivity as explained below.

According to an embodiment of the present invention, the input opticalbeam can comprise K wavelength components (K being an integer of atleast a value of two), such that the at least one optically absorbingarea within or disposed on each of the substrates is configured toabsorb M (M being an integer between 1 and K−1) pre-selected componentsof the K wavelength components and to propagate a selected component outof the K wavelength components, e.g., substantially without an opticalintensity attenuation due to absorption. Then the output optical beamfrom each substrate substantially comprises the selected component outof the K wavelength components. It is noted that the diffractiongratings in each substrate are optimized (e.g., by choosing theappropriate period of the periodic lines) for the wavelength componentto be selected by that grating.

Thus, each of the N stacked substrates is configured to expand an exitpupil of substantially only one component out of said K wavelengthcomponents of the input optical beam such that all said K wavelengthcomponents merge together having substantially the same direction andlocation in an output of said optical device, therefore providing acolor separation using the N exit pupil expanders that use a pluralityof diffractive elements for expanding the exit pupil, e.g., of a displayin the electronic device for viewing.

In one possible embodiment, N can be equal to K, i.e., each substrateonly can output one wavelength (color) component. In another embodiment,the optically absorbing area for any substrate out of the N substratescan be configured to absorb all K−1 wavelength components except onlyone selected wavelength components band to be provided in the outputoptical beam from that substrate. Also different substrates can beconfigured to absorb a different number of wavelength componentsdepending on a specific system design.

According to a further embodiment of the present invention, in case of atwo-dimensional exit pupil expansion, each substrate can comprise atleast one intermediate diffractive element (two intermediate diffractiveelements for the biocular case) such that the at least part of the inputoptical beam diffracted in the at least one diffractive element is firstcoupled to the at least one intermediate diffractive element, which thencouples, using a further diffraction in the at least one intermediatediffractive element, the at least part of said diffracted optical beamto the at least one further diffractive element, which then provides atwo-dimensional exit pupil expansion of said input optical beam. Theintermediate diffractive element can have an odd number of first orderdiffractions or an even number of further first order reflections asknown in the art and, e.g., described by T. Levola in “DiffractiveOptics for Virtual Reality Displays”, SID Eurodisplay 05, Edinburg(2005), SID 02 Digest, Paper 22.1.

Furthermore, according to an embodiment of the present invention, the atleast one optically absorbing area within or disposed on each substratecan be configured using at least one approach or a combination thereofout of:

-   -   a) an absorbing tint spread throughout a volume of each        substrate,    -   b) an absorbing tint spread throughout a thickness of each        substrate only in areas of each substrate between the at least        one diffractive element and at least one further diffractive        element (i.e., between the in-coupling and the out-coupling        diffraction gratings),    -   c) an absorbing tint spread throughout a resin used to make the        at least one diffractive element, the at least one further        diffractive element, and/or the intermediate grating,    -   d) an absorbing coating, having substantially the same index of        refraction as each substrate, disposed on each substrate in an        area substantially between the at least one diffractive element        and at least one further diffractive element (i.e., between the        in-coupling and the out-coupling diffraction gratings), etc.

In a further embodiment of the present invention, each substrate can beconfigured to have an additional optically absorbing layer disposed onor adjacent to a surface of that substrate, wherein the surface being asecond surface of that substrate through which the input optical beampropagates after being received by the substrate, such that the selectedcomponent out of the K wavelength components selected by that substrateis substantially absorbed in the additional optically absorbing layer,thus preventing said selected wavelength component from entering thenext substrate.

One practical example for implementing embodiments of the presentinvention can be a color separation for an RGB (red, green, blue) gamut.In this case, the first substrate in the stack can “select” a short bluewavelength component and “absorb” the green and red, the secondsubstrate can “select” a green component, and “absorb” the blue and red,and finally the third substrate in the stack can “select” a long redwavelength component and “absorb” the green and blue. Moreimplementation examples are provided in FIGS. 2 through 6, according tovarious embodiments of the present invention.

FIGS. 1 a and 1 b show schematic representations of a virtual realitydisplay (a cross sectional view shown in FIG. 1 a) having an exit pupilexpander system 10 a with stacked diffractive exit pupil expanders 10-1,10-2, etc. as shown in a cross sectional view in FIG. 1 b within-coupling gratings 12-1 and 12-2, etc. and out-coupling gratings 14-1,14-2, 16-1 and 16-2 for the biocular case. For simplicity, the inputoptical beam has two wavelength components 20 (γ₁) and 22 (γ₂). In anapproach shown in FIG. 1 b, both components 20 and 22 are coupled (seecorresponding optical beams 20-1 a, 22-1 a, 20-2 a, 22-2 a, and unwantedoptical beams 20-1 c, 22-1 c, 20-2 c, 22-2 c, respectively) to thecorresponding out-coupling gratings 14-1, 14-2, 16-1 and 16-2 in each ofthe substrates 10-1 and 10-2 as shown in FIG. 1 b, such that bothwavelength components 20 and 22 are provided in the output optical beamby each of the substrates 10-1 and 10-2 in spite of the fact that theout-coupling diffraction gratings 14-1 and 16-1 are optimized only forthe wavelength component 20 and the out-coupling diffraction gratings14-2 and 16-2 are optimized only for the wavelength component 22. Thismixing of colors deteriorates the quality of the image provided by theexit pupil expander system 10. The example of the virtual realitydisplay of FIG. 1 a with the stacked diffractive exit pupil expanders,shown in FIG. 1 b, can be used for applying embodiments of the presentinvention. FIGS. 2 through 6 provide different implementation examplesfor eliminating color mixing, according to embodiments of the presentinvention.

FIGS. 2 a and 2 b show examples among others of schematicrepresentations (cross-sectional views) demonstrating a color separationin an exit pupil expander system 10 using absorbing tint through avolume of the substrates 10-1 and 10-2 (FIG. 1 a) and through athickness of selected areas 10-1 a, 10-2 a, 10-1 b and 10-2 b of thesubstrates 10-1 and 10-2 between the in-coupling gratings 12-1 and 12-2,and the out-coupling gratings 14-1, 14-2, 16-1 and 16-2, respectively,as shown in FIG. 2 b, according to embodiments of the present invention.The absorbing tint in the substrate 10-1 is optimized for absorbing thewavelength component 22 while being transparent to the wavelengthcomponent 20, and the absorbing tint in the substrate 10-2 is optimizedfor absorbing the wavelength component 20 while being transparent to thewavelength component 22. Thus, each of the substrates 10-1 and 10-2provides only one wavelength component: the optical beams 20-1 b and20-2 b representing the wavelength component 20 are provided by thesubstrate 10-1, and the optical beams 22-1 b and 22-2 b representing thewavelength component 22 are provided by the substrate 10-2,respectively.

FIGS. 3 a and 3 b show examples among others of schematicrepresentations (cross-sectional views) of a color separation in an exitpupil expander system 10 using the absorbing tint spread throughout aresin used in the in-coupling gratings 12-1 and 12-2 and/or in theout-coupling gratings 34-1, 34-2, 36-1 and 36-2, according toembodiments of the present invention. In FIG. 3 a, the absorbing tint inthe diffraction gratings 34-1, 36-1 of the first substrate 10-1 isoptimized for absorbing the wavelength component 22 while beingtransparent to the wavelength component 20, and the absorbing tint inthe diffraction gratings 34-2, 36-2 of the first substrate 10-2 isoptimized for absorbing the wavelength component 20 while beingtransparent to the wavelength component 22, thus eliminating unwantedoptical beams 20-1 c, 22-1 c, 20-2 c, 22-2 c by absorption in theout-coupling gratings 34-1, 34-2, 36-1 and 36-2. It is noted that thein-coupling grating 12-2 provides an initial attenuation of the unwantedoptical beams 20-1 c and 20-2 c as shown in FIG. 3 a. If the in-couplinggrating 12-2 can provide enough attenuation of the unwanted opticalbeams 20-1 c and 20-2 c, then the out-couplings 14-2 and 16-2 do notneed to be tinted at all as shown in FIG. 3 b. This provides anadvantage that the out-coupling beams 20-1 b and 20-2 b from the firstsubstrate 10-1 are not attenuated by the absorbing tint in thediffraction gratings 14-2 and 16-2.

FIG. 4 a shows an example among others of schematic representations(cross-sectional views) demonstrating a color separation in an exitpupil expander system 10 using the absorbing tint spread throughout aresin in the out-coupling gratings 34-1, 34-2, 36-1 and 36-2 such thatthe unwanted optical beams 20-1 c, 22-1 c, 20-2 c, 22-2 c are eliminatedby absorption in the out-coupling gratings 34-1, 34-2, 36-1 and 36-2,respectively. In addition (optionally), an optically absorbing layer 40disposed on or adjacent to a surface of the substrate 10-1 across thein-coupling grating 12-1 can absorb the wavelength component 20 forminimizing the presence of the unwanted optical beams 20-1 c and 20-2 c.If the in-coupling grating 12-2 can provide enough attenuation of theunwanted optical beams 20-1 c and 20-2 c, then the out-couplings 14-2and 16-2 do not need to be tinted at all as shown in FIG. 4 b. Thisprovides an advantage that the out-coupling beams 20-1 b and 20-2 b fromthe first substrate 10-1 are not attenuated by the absorbing tint in thediffraction gratings 14-2 and 16-2, respectively.

FIG. 5 a shows an example among others of schematic representations(cross-sectional views) demonstrating a color separation in an exitpupil expander system 10 using the absorbing tint spread throughout theresin in the out-coupling gratings 34-1 and 36-1 of the first substrate10-1 such that the unwanted optical beams 22-1 c and 22-2 c areeliminated by absorption in the out-coupling gratings 34-1 and 36-1,respectively. In the second substrate 10-2, absorbing coatings 42-1 and42-2 having substantially the same index of refraction as the substrate10-2 and disposed on the substrate 10-2 in areas between the in-couplinggrating 12-2 and the out-coupling gratings 14-2 and 16-2, respectively,according to an embodiment of the present invention. These absorbingcoatings 42-1 and 42-2 substantially absorb the unwanted optical beams20-1 c and 20-2 c in the second substrate 10-2 acting as a boundary forthe total internal reflection. In the example of FIG. 5 b, instead ofusing the absorbing tint spread throughout the resin in the out-couplinggratings (as shown in FIG. 5 a), further absorbing coatings 44-1 and44-2 having substantially the same index of refraction as the substrate10-1 and disposed on the substrate 10-1 in areas between the in-couplinggrating 12-1 and the out-coupling gratings 14-1 and 16-1, respectively,according to a further embodiment of the present invention. Theseabsorbing coatings 44-1 and 44-2 substantially absorb the unwantedoptical beams 22-1 c and 22-2 c in the first substrate 10-1 acting as aboundary for the total internal reflection as well.

It is noted that tinting is a known art, e.g., see U.S. Pat. No.6,464,733 “Tinting plastic articles” by C. U. Ryser. For selectivetinting of the substrate (see FIG. 2 b), the parts that needs to betinted can be tinted separately. Then these parts can be molded togetherwith clear plastics. The tinted and not tinted parts should have thesame refractive index, which is usually the case, if the same materialis used thoroughly, because the tinting does not usually alter therefractive index. Also radiation methods may be used for creating colorcenters in the material to alter the spectral absorption. Moreover, thediffraction gratings can be formed using a UV (ultraviolet) curableresin, which is tinted which, as known in the art.

FIG. 6 shows an example of a schematic representation of an electronicdevice, having the exit pupil expander (EPE) system 10, according to anembodiment of the present invention.

The exit pupil expander (EPE) 10, 10 a or 10 b can be used in anelectronic (portable) device 100, such as a mobile phone, personaldigital assistant (PDA), communicator, portable Internet appliance,hand-hand computer, digital video and still camera, wearable computer,computer game device, specialized bring-to-the-eye product for viewingand other portable electronic devices. As shown in FIG. 6, the portabledevice 100 has a housing 210 to house a communication unit 212 forreceiving and transmitting information from and to an external device(not shown). The portable device 100 also has a controlling andprocessing unit 214 for handling the received and transmittedinformation, and a virtual display system 230 for viewing. The virtualdisplay system 230 includes a micro-display or an image source 192 andan optical engine 190. The controlling and processing unit 214 isoperatively connected to the optical engine 190 to provide image data tothe image source 192 to display an image thereon. The EPE 10, accordingto the present invention, can be optically linked to an optical engine190.

Furthermore, the image source 192, as depicted in FIG. 6, can be asequential color LCOS (Liquid Crystal On Silicon) device, an OLED(Organic Light Emitting Diode) array, an MEMS (MicroElectro MechanicalSystem) device or any other suitable micro-display device operating intransmission, reflection or emission.

Moreover, the electronic device 100 can be a portable device, such as amobile phone, personal digital assistant (PDA), communicator, portableInternet appliance, hand-held computer, digital video and still camera,wearable computer, computer game device, specialized bring-to-the-eyeproduct for viewing and other portable electronic devices. However, theexit pupil expander, according to the present invention, can also beused in a non-portable device, such as a gaming device, vending machine,band-o-matic, and home appliances, such as an oven, microwave oven andother appliances and other non-portable devices.

It is to be understood that the above-described arrangements are onlyillustrative of the application of the principles of the presentinvention. Numerous modifications and alternative arrangements may bedevised by those skilled in the art without departing from the scope ofthe present invention, and the appended claims are intended to coversuch modifications and arrangements.

What is claimed is:
 1. An apparatus, comprising: a substrate of opticalmaterial having a first surface and a second surface; at least onediffractive element disposed on the first or the second surface of saidsubstrate and configured to receive an input optical beam; at least onefurther diffractive element disposed on the first or the second surface;and at least one optically absorbing area within said substrate, whereinthe at least one diffractive element is configured to diffract at leastpart of the input optical beam to provide at least one diffractedoptical beam substantially within the first and the second surfaces, andsaid at least one further diffractive element is configured to diffractat least part of the at least one diffracted optical beam such that atleast part of the at least one diffracted optical beam is coupled out ofthe substrate, thus said at least one further diffractive element isconfigured to provide an output optical beam out of said substrate withan expanded exit pupil in one or two dimensions, wherein said inputoptical beam comprises K wavelength components and said at least oneoptically absorbing area is configured to absorb M pre-selectedcomponents out of the K wavelength components of said at least onediffracted optical beam and to propagate a selected component out of theK wavelength components of said at least one diffracted optical beam,such that the output optical beam comprises said selected component outof the K wavelength components, wherein K is an integer of at least avalue of two and M is an integer between 1 and K−1.
 2. The apparatus ofclaim 1, wherein said apparatus comprises N stacked substrates ofoptical material separated by a gap, N being an integer of at least avalue of two, and said substrate being one of said N substrates, whereineach of said N stacked substrates is configured to expand an exit pupilsubstantially of only one component out of said K wavelength componentsof the input optical beam such that all said K wavelength componentsmerge together having substantially the same direction and location inan output of said apparatus.
 3. The apparatus of claim 2, wherein N=K.4. The apparatus of claim 1, wherein M=K−1.
 5. The apparatus of claim 1,wherein the substrate is configured to provide, as the output opticalbeam, an optical beam substantially comprising only said selectedcomponent out of the K wavelength components.
 6. The apparatus of claim1, wherein said at least one optically absorbing area is configuredusing at least one of: an absorbing tint spread throughout a volume ofthe substrate, an absorbing tint spread throughout a thickness of thesubstrate only in areas of the substrate between the at least onediffractive element and at least one further diffractive element, anabsorbing tint spread throughout a resin used to make said at least onediffractive element and an absorbing tint spread throughout the resinused to make said at least one further diffractive element.
 7. Theapparatus of claim 1, further comprises: a further substrate of opticalmaterial having a further first surface and a further second surface,said further substrate being stacked substantially in parallel with saidsubstrate but maintaining a gap with said substrate; at least oneadditional diffractive element disposed on the further first or thefurther second surface of said further substrate and configured toreceive a portion of the input optical beam which propagates through thesubstrate into the further substrate; at least one still furtherdiffractive element disposed on the further first or the further secondsurface; and at least one further optically absorbing area within ordisposed on said further substrate, wherein the additional diffractiveelement is configured to diffract at least further part of the portionof the input optical beam to provide at least one further diffractedoptical beam substantially within the further first and the furthersecond surfaces, and said at least one still further diffractive elementis configured to diffract at least further part of the at least onefurther diffracted optical beam such that at least further part of theat least one further diffracted optical beam is further coupled out ofthe further substrate, thus said at least one still further diffractiveelement is configured to provide a further output optical beam out ofsaid further substrate with an expanded exit pupil in one or twodimensions, said further output optical beam having substantially thesame direction and location as the output optical beam, wherein said atleast one further optically absorbing area is configured to absorb Ppre-selected components of the K wavelength components and to propagatea further selected component out of the K wavelength components, suchthat the further output optical beam comprises said further selectedcomponent out of the K wavelength components, wherein P is an integerbetween 1 and K−1.
 8. The apparatus of claim 7, wherein P=K−1.
 9. Theapparatus of claim 7, wherein said at least one further opticallyabsorbing area is configured using at least one of: an absorbing tintspread throughout a volume of the further substrate, an absorbing tintspread throughout a thickness of the further substrate only in areas ofthe further substrate between the at least one additional diffractiveelement and at least one still further diffractive element, an absorbingtint spread throughout a resin used to make said at least one additionaldiffractive element and an absorbing tint spread throughout the resinused to make said at least one still further diffractive element. 10.The apparatus of claim 7, wherein said gap is an air gap.
 11. Theapparatus of claim 7, wherein the substrate is configured to provide, asthe output optical beam, an optical beam substantially comprising onlysaid selected component out of the K wavelength components and thefurther substrate is configured to provide, as the further outputoptical beam, an optical beam substantially comprising only saidselected further component out of the K wavelength components and thesubstrate and the further substrate are configured to provide the outputoptical beam and the further output optical beam such that the outputoptical beam and the further output optical beam merge together havingsubstantially the same direction and location.
 12. The apparatus ofclaim 1, wherein said substrate is configured to have an additionaloptically absorbing layer disposed on or adjacent to a surface of saidsubstrate, said surface being a second surface through which the inputoptical beam propagates, such that said additional optically absorbinglayer is configured to substantially absorb said selected component outof the K wavelength components.
 13. The apparatus of claim 1, furthercomprises at least one intermediate diffractive element such that the atleast one diffractive element is configured to diffract the at leastpart of the input optical beam to first couple the at least part of theinput optical beam to said at least one intermediate diffractiveelement, which is configured to couple, using a further diffraction insaid at least one intermediate diffractive element, said at least partof said diffracted optical beam to said at least one further diffractiveelement, which is configured to then provide a two-dimensional exitpupil expansion of said input optical beam.
 14. An apparatus as claimedin claim 1, wherein the apparatus is an Exit Pupil Expander.
 15. Anelectronic device comprising an Exit Pupil Expander as claimed in claim14.
 16. The electronic device as claimed in claim 15, wherein theelectronic device is a mobile terminal.
 17. A method, comprising:receiving an input optical beam by at least one diffractive elementdisposed on a first or a second surface of a substrate, said inputoptical beam comprises K wavelength components, wherein K is an integerof at least a value of two; diffracting at least part of the inputoptical beam in the at least one diffractive element to provide at leastone diffracted optical beam substantially within the first and secondsurfaces; absorbing M pre-selected components out of the K wavelengthcomponents of said at least one diffracted optical beam in at least oneoptically absorbing area within said substrate and propagating aselected component out of the K wavelength components of said at leastone diffracted optical beam, wherein M is an integer between 1 and K−1;and coupling at least part of the at least one diffracted optical beamout of the substrate by diffraction in said at least one furtherdiffractive element, thus providing an output optical beam out of saidsubstrate with an expanded exit pupil in one or two dimensions, whereinthe output optical beam comprises said selected component out of the Kwavelength components.
 18. The method of claim 17, wherein the outputoptical beam provided by the substrate substantially comprises only saidselected component out of the K wavelength components.
 19. The method ofclaim 17, wherein said at least one optically absorbing area isconfigured using at least one of: an absorbing tint spread throughout avolume of the substrate, an absorbing tint spread throughout a thicknessof the substrate only in areas of the substrate between the at least onediffractive element and at least one further diffractive element, anabsorbing tint spread throughout a resin used to make said at least onediffractive element and an absorbing tint spread throughout the resinused to make said at least one further diffractive element.
 20. Anapparatus, comprising: at least one means for diffraction, for receivingan input optical beam by at least one means for diffraction disposed ona first or a second surface of a substrate, said input optical beamcomprises K wavelength components, wherein K is an integer of at least avalue of two, and for diffracting at least part of the input opticalbeam in the at least one means for diffraction to provide at least onediffracted optical beam substantially within the first and secondsurfaces; at least one absorbing means within said substrate, forabsorbing M pre-selected components out of the K wavelength componentsof said at least one diffracted optical beam by said at least oneabsorbing means and propagating a selected component out of the Kwavelength components of said at least one diffracted optical beam,wherein M is an integer between 1 and K−1; and at least one furthermeans for diffraction, for coupling at least part of the at least onediffracted optical beam out of the substrate by diffraction in said atleast one further means for diffraction, thus providing an outputoptical beam out of said substrate with an expanded exit pupil in one ortwo dimensions, wherein the output optical beam comprises said selectedcomponent out of the K wavelength components.
 21. The apparatus of claim20, wherein the apparatus is an exit pupil expander.